The present invention relates to an apparatus for converting aspect ratio and number of scanning lines of a video signal used in converting a broadcasting signal of HDTV called as the next generation video apparatus into a TV signal capable of watching through the present NTSC standard TV set, and particularly to a converting apparatus for converting mode corresponding to aspect ratio and number of scanning lines of a video signal suited to NTSC standard such as conversion of a HDTV video image having aspect ratio of 16 to 9 (width-to-height) and scanning lines above 1050 into a video signal having aspect ratio of 3 to 4 and scanning lines of 525.
Conventionally, in converting a HDTV video signal having aspect ratio of 16 to 9 and scanning lines above 1050 into a video signal which can be watched through NTSC standard TV, because aspect ratio and number of scanning lines of a picture screen are different between two standards, the fixed conversion rate is determined on the basis of reference rate and aspect ratio and number of scanning lines are converted according to the rate.
Therefore, the conversion mode of several formats is determined according to how did the conversion rate determined.
For example, in converting TV video signal of MUSE standard which is a HDTV broadcast standard of Japan into a TV video signal for NTSC standard, a zoom-up mode conversion method and a wide mode conversion method have been used as shown in FIG. 1.
In a zoom-up mode conversion method, a video signal of HDTV having aspect ratio of 16 to 9 and scanning lines of 1125 is cut off on the order of 15% of both sides and 525 lines are picked out to conform to aspect ratio of 3 to 4 on the basis of the center of a video image.
In a wide mode conversion method, the whole picture screen of HDTV having aspect ratio of 16 to 9 is reduced to be maintained said HDTV aspect ratio so that scanning lines in which a video image is not exist in the upper and lower picture screen of NTSC standard TV having aspect ratio of 3 to 4 are remained on the order of 75 lines and 375 lines of center are converted.
As above-mentioned, in converting a HDTV signal into a NTSC standard TV signal, cut off of a HDTV video signal in both sides and number of scanning lines become changed according to the conversion rate due to the reference rate and therefore it is possible to convert several modes according to cut off rate of both sides of a HDTV video signal and number of converting scanning lines.
On the other hand, if cut off rate of both sides becomes decreased, number of converting scanning lines is decreased and number of scanning lines in which a video signal is not exist in the upper and lower of a NTSC TV picture screen become increased.
Therefore, in converting a HDTV video signal into a NTSC standard TV video signal it is suited for converting more than two modes such as a zoom-up mode and a wide mode.
As above-mentioned the basic principle of conversion method of aspect ratio and number of scanning lines of a video signal is that a number of pixel suited to each modes converted from a HDTV video signal having aspect ratio of 16 to 9 and scanning lines above 1050 is written into line memory, field memory or frame memory by clock rate(For example, sampling rate) suited to a HDTV video signal processing. Then after the fixed time, video signal is read out from said memory by clock rate for NTSC standard TV video signal processing which is determined according to conversion mode.
Therefore, in converting aspect ratio and number of scanning lines, the write and read control operation are changed according to conversion mode.
In the prior art, due to aforesaid cause, the separate memory control circuits which are necessary for write and read of video image data corresponding to every conversion modes are configured as shown in FIG. 2.
Therefore, an apparatus for converting aspect ratio and number of scanning lines of a video signal according to the prior art has disadvantages in that if a conversion mode is more than two, hardware configuration of write and read control circuits of a video memory is increased in proportion to the preferred number of conversion mode.
Aforesid converting technique is disclosed in Japanese Laid Open Patent Application No. 1-202984.